The Back-up Plan director Alan Poul talks about the importance of casting the right leading man:

We spent a long time looking for Stan. It’s especially difficult, because the movie belongs more to Zoe. I knew from the moment when I read the script that if we didn’t cast Stan correctly, the movie would collapse because Stan would seem like a wimp and a loser.

So we looked very hard all around and Alex has that combination of qualities of a kind of effortless masculinity that doesn’t take itself too seriously and he also, I think, has enough charisma that he can hold the screen with Jennifer.